One idea is to use a focused light beam to take advantage of the fact that photos have momentum even when they have no mass at rest.

The photons would be fired at the craft using a laser, which would propel the sail forwards. The effect increases over time, meaning the craft could be accelerated to enormous speeds without any rockets or propellant of its own.

The idea has already been used in space, with the Japanese Ikaros probe employing a 46-foot version of the sail in 2010.

But using the sail to get to other planets, let alone stars, presents some enormous problems for current generation technology.

For one, the sail would have to be enormous. Les Johnson, deputy manager of the Advanced Concepts Office, told Space.com that "the physics tells us it's going to be the size of Texas".

It would also require a laser that could be targeted "more accurately than we can point anything today", and a "power output basically comparable to the whole of humanity".

Then there are the distances involved. Even if the craft could reach a few percent of the speed of light, it would take hundreds of years to reach other stars. Current spacecraft would need 40,000 years to reach our nearest solar neighbour, Alpha Centauri, which is just 4.3 light years away.

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Still, Johnson told Space.com that he is convinced we will get there eventually - just not today.

The idea is that a giant version of the sail could be deployed to help reach the stars within a useful time frame.

"We're going to run into the problem of the limitations of the solar system eventually," he said. "So the next step will be, there's a whole galaxy out there. It's too big a step for us to take now, but I would like to think that several generations in the future, that'll just be the next logical step."